The present invention relates to a method for measuring a pressure applied between opposing surfaces using a pressure measuring film, particularly a method for measuring a pressure higher than a measurable pressure range of the pressure measuring film. The present invention also relates to a pressure measurement apparatus for direct use in carrying out the method.
There is known a method in which a pressure measuring film is disposed between a pair of pressing surfaces to determine the pressure from changes in color formed on the pressure measuring film. The color development depends on the magnitude of the pressure applied. As such pressure measuring film, there is exemplified xe2x80x9cPrescalexe2x80x9d (tradename) presented by Fuji Photo Film Co., Ltd.
The Prescale comprises microcapsules containing a color former and a color developer coated on the same surface of a support, or on different supports. The microcapsules at the portion subjected to pressure is destroyed to cause the color former released from the ruptured microcapsules to react to the developer so as to develop a predetermined color (e.g., red) by chemical reaction. There are a type in which a microcapsulated color former and a developer are coated on one sheet of support (referred to as the mono-sheet type), and a type in which they are coated to different supports and two supports are overlaid for use (referred to as the two-sheet type).
In the pressure measuring film, since the developed color density increases by pressure increase, the pressure applied can be determined by measuring the developed color density. The pressure measuring film is able to measure different pressure ranges by using microcapsules which have different wall thicknesses to be ruptured upon application of different pressure levels. A plurality of types of pressure measuring films for different measurable pressure ranges are prepared, and the film with an appropriate pressure range is selected and used depending on the pressure to be measured.
However, when the pressure level to be measured is higher than the measurable pressure range of the pressure measuring film, the pressure cannot correctly be measured with this pressure measuring film. For example, the cylinder block and cylinder head of a reciprocating engine (internal combustion engine) with a gasket disposed therebetween are clamped by a cylinder head bolt. When a bonding pressure between the cylinder block and the cylinder head is measured by the pressure measuring film, this pressure becomes larger than the pressure range of the pressure measuring film for the highest pressure use.
In this case, the measurable pressure range of an available pressure measuring film (Prescale) for high-pressure is 50 to 130 MPa (additionally, 1 MPa is nearly equal to 10.2 kgf/cm2), while the bonding pressure of the cylinder block and cylinder head is about 1.8 ton/cm2 (nearly equal to 176.5 MPa). Therefore, even when the pressure measuring film for high-pressure is used, it can only be judged that the detected pressure is higher than the maximum value (130 MPa) of the measurable pressure range. The color density at the portion to which such high pressure is applied reaches up to the maximum density and saturates in this density level. Accordingly, the actual maximum bonding pressure cannot correctly be measured, which is a large obstruction during development of the engine and gasket.
The present invention has been accomplished in consideration of the above-described situations, and a first object thereof is to provide a pressure measurement method which can measure a pressure higher than a measurable pressure range using a pressure measuring film. Moreover, a second object is to provide a pressure measurement apparatus for direct use in carrying out this method.
To attain the first object, according to the present invention, there is provided a method for measuring a pressure applied between opposing first and second surfaces using a pressure measuring film, the first surface being flatter than the second surface, the second surface having a contact portion to contact with the first surface in point contact or linear contact, comprising steps of:
providing the pressure measuring film between said first and second surfaces, the pressure measuring film causing a pressurized portion thereof to develop a color;
disposing an elastic sheet between said second surface and said pressure measuring film;
applying the pressure between said first and second surfaces while holding said pressure measuring film and said elastic sheet therebetween;
detecting a colored image formed on said pressure measuring film to obtain a pressure distribution curve; and
determining a maximum pressure applied between said first and second surfaces based on the pressure distribution curve;
wherein the pressure applied to said pressure measuring film is decreased as compared with the case in which the elastic sheet is not used.
The point or linear contact portion on the second surface does not restrictedly mean the point or linear shape, and of course has a certain degree of area. For example, for the gasket disposed between the cylinder block and cylinder head of the engine, a protrusion disposed along a circle surrounding a cylinder periphery forms the linear contact portion. And the top surface of the gasket including the protrusion constitutes the second surface of the present invention. The bottom surface of the cylinder head onto which the protrusion is abutted and pressed constitutes the first surface of the present invention. Specifically, one of the pressing surfaces disposed opposite to each other is flatter than the other, and this one (i.e., the first surface) is abutted and contacted with the other pressing surface (i.e., the second surface) in an acute manner. For example, when a sheet is held between both pressing surfaces, and when the contact area of the other pressing surface and the sheet is smaller than the contact area of one flatter pressing surface and the sheet, the pressure measurement method of the present invention can be applied.
As the pressure measuring film for use herein, a film can be used which is obtained by forming a layer of a microcapsulated color former and a layer of a color developer on a support (film). By application of pressure, the microcapsules are destroyed to release the color former from the ruptured microcapsules. The released color former is adsorbed to the developer and a color is developed by chemical reaction. For example, xe2x80x9cPrescalexe2x80x9d (tradename) sold by Fuji Photo Film Co., Ltd. can be used.
By reading a colored image formed on the pressure measuring film, the density distribution on a line crossing the colored image which corresponds to the contact portion of point or linear contact is measured to obtain the pressure distribution along the crossing line. The maximum pressure PH between the pressing surfaces can be obtained by using the area SE surrounded by the pressure distribution curve and the contact length L of the contact portion. Here, the area SE surrounded by the pressure distribution curve is considered to correspond to the work amount (pressurexc3x97compression amount) by pressing, or to the amount obtained by integrating this work amount by a coefficient for correcting the influence by an elastic sheet. Consequently, the maximum pressure applied between the pressing surfaces can be determined using this area SE and contact length L. For example, the maximum pressure PH may be determined so that the following relationship is satisfied:
(contact length)xc3x97(maximum pressure)=area
(Lxc3x97PH=SE)
The contact length L used herein may be detected from the colored image obtained when the same pressure measuring film (second pressure measuring film) is held and pressed between the pressing surfaces without using any elastic sheet. The contact length L may also be obtained as follows. The pressure measuring films of the same standard are held between both surfaces of the elastic sheet and two pressing surfaces, and the contact length L is obtained from the colored image formed on the pressure measuring film (second pressure measuring film) which directly contacts to the point/linear contact portion of the pressing surface (second surface).
The pressure measuring film (second pressure film) having the same characteristics as that of the above-described pressure measuring film (first pressure measuring film) is pressed between the pressing surfaces without using the elastic sheet, and a second pressure distribution curve F may be obtained from the resulting color development state. When the pressure distribution curve is obtained from the colored image formed on the pressure measuring film by using the elastic sheet as described above and the thus obtained curve is used as a first pressure distribution curve E, the aimed maximum pressure can be obtained by enlarging or extrapolating the second pressure distribution curve toward the maximum value so that the area SF surrounded by the enlarged second pressure distribution curve substantially becomes the same as the area SE of the first pressure distribution curve.
Alternatively, the respective pressure measuring films may be held and pressed between both sides of the elastic sheet and two pressing surfaces. The pressure measuring film coming in contact with the point/linear contact portion of the second pressing surface is used as the second pressure measuring film, and the other film can be used as the first pressure measuring film. In this arrangement, since the necessary pressure distribution curves can be obtained by once pressing the first and second pressure measuring films between the pressing surfaces, measurement precision is enhanced.
A hard rubber sheet may be used as the elastic sheet, but when the pressure is particularly large, hard glass epoxy resin having less elastic deformability than that of hard rubber may be used. If between both the pressing surfaces, the pressure applied from the second pressing surface including a more acutely protruded contact portion is dispersed in the elastic sheet, and the pressure transmitted and applied to the pressure measuring film is reduced, such sheet can be used as the elastic sheet in the pressure measurement method of the present invention.
To attain a second object, there is provided a pressure measurement apparatus for use in the method described above, the apparatus comprising:
a scanner for reading the colored image formed on a pressure measuring film to which a pressure is applied to develop color;
a first memory for storing the image read by the scanner;
an image cutout section for obtaining an image on a line crossing a contact portion of point contact or line contact from the read image;
a converter for obtaining a pressure distribution from the image cut out by the image cutout section;
a second memory for storing the pressure distribution obtained by the converter;
a processor for using the pressure distribution stored in the second memory to obtain a maximum pressure applied between surfaces; and
an output means for outputting an operation result obtained by the processor.